1. Field of the Invention
This invention relates to low pressure breathing regulators for use in aircraft breathing gas systems.
2. Description of the Prior Art
In many present day aircraft, oxygen-enriched air is supplied as breathing gas for an aircrew member by an on-board oxygen generating system (OBOGS) comprising a molecular sieve oxygen generating system (MSOGS) arranged to deliver oxygen-enriched air of desired oxygen concentration value by adsorbing nitrogen from air fed to the system. Oxygen-enriched air produced by either system is delivered to an aircrew breathing mask by way of a demand valve breathing regulator. A problem was found to exist with respect to demand valve operation in a breathing regulator suitable for accommodating the lower range of oxygen-enriched air pressure available from a MSOGS, particularly at the lower end towards 70 kPa (10 psi).
This problem was overcome by a breathing regulator disclosed in EP-A No. 0,078,644 (Normalair-Garrett) which embodies a diaphragm arranged for sensing breathing demand and actuating, via a lever, a pressure balanced demand valve. The diaphragm separates a demand-pressure sensing chamber from a breathing-pressure control chamber having communication by way of an aneroid valve with a cabin-pressure sensing chamber. A controlled bleed is provided from the demand-pressure sensing chamber to the breathing-pressure control chamber, in the particular embodiment of EP-A-0,078,644 the bleed being by way of an orifice in the diaphragm, and pressure in the breathing-pressure control chamber is controlled by the aneroid valve which allows gas to pass from the breathing-pressure control chamber to the cabin-pressure sensing chamber from which it is discharged to the cabin by way of an outlet in the cabin-pressure sensing chamber. When the-aircraft operating altitude exceeds 12,000 meters (40,000 feet) the aneroid valve expands to increasingly restrict the flow of gas from the breathing-pressure control chamber. This causes the pressure in the breathing-pressure control chamber to increase thereby increasing the pressure of the breathing gas at the regulator outlet and hence in a breathing mask connected to the regulator outlet. This ensures that breathing gas is supplied at a pressure greater than aircraft cabin ambient pressure so that the minimum critical oxygen pressure is maintained in the lungs of the aircrew member breathing the gas. This is referred to in the art as positive pressure breathing.
It is known that protection against rapid and high increases of G loads, e.g. 3.5G to 9G, such as are experienced by an aircrew member during aircraft maneuvers where large accelerative forces occur, is enhanced by positive pressure breathing. The increase in breathing pressure causes an approximately equal increase in heart level blood pressure, thereby increasing the flow of blood to the brain.
There is a requirement, therefore, for a breathing regulator suitable for use with breathing gas delivered by a MSOGS and which will provide positive pressure breathing to aid in protecting an aircrew member against the effects of increasing G loads experienced during highly accelerative maneuvers of his aircraft irrespective of the altitude at which the aircraft is operating.
Also, it has been found that in certain conditions, such as when sudden and rapid increases of G load occur, an aircrew member will make a rapid deep inhalation to fill his lungs. In so doing he may draw all the oxygen-enriched air from the outlet and the demand-pressure sensing chamber of the regulator disclosed in EP-A-0,078,644. This causes the diaphragm separating the demand-pressure sensing chamber from the breathing pressure control chamber to move towards the bottom of the demand-pressure sensing chamber and significantly reduces the pressure in the breathing-pressure control chamber. This pressure will only rebuild slowly during which period the regulator is rendered inoperative.